1. Field of the Invention
The present invention relates to network management. More particularly, the present invention relates to a network delay shaping system and method for backhaul of wireless networks.
2. Background Information
Wireless mesh backhaul, particularly those using frame-based protocols such as I.E.E.E. 802.16, WiMAX, or the like, are starting to be employed to wirelessly transport data from multiple cellular radio sites, or base transceiver stations (BTS), to a single or multiplicity of redundant network access or egress points. The data from these multiple BTS sites is then aggregated for wire-line transport to one or more centralized offices (CO) to be processed and switched into the larger regional and national networks. Aggregation enables economies of scale, because the price per wire-line transported bit is reduced as more bits are leased. For example, in conventional networks, most cell BTS sites are maintained using one to three digital signal 1 (DS1, e.g., T1 or E1) circuits. With aggregation, DS3 circuits capable of carrying 28 DS1 circuits can be leased at a rate equivalent to eight DS1s. Wireless backhaul solutions therefore become cost effective when just a few BTS cell sites carrying eight or more DS1 circuits are aggregated. The same economies of scale are true for packet based networks, where a single 100 Mbps data pipe can be leased for the equivalent of four 10 Mbps data pipes.
Wireless transport, especially frame based wireless protocols such as WiMAX and the like, have significantly more delay that wired networks. WiMAX networks, with frame sizes of 2.5 ms, 4 ms, 5 ms, 7.5 ms, 10 ms, and 20 ms, usually add delays that are on the order of 1.25 to 1.75 frames for a single wireless hop, in addition to the “speed of light” delays proportional to distance from the BTS to the CO site. Wired transport backhaul networks delays are driven mostly by “speed of light” (or equivalent speed of electrical propagation in wire) delays. Both wired and wireless networks add lesser delays for “line rate” switching of the packets or circuits. These delays are typically of the order of tens to hundreds of microseconds and have a minimal effect on the overall network delay.
The code division multiple access (CDMA) network protocol requires strict relative delays between BTS sites to enable seamless or “soft” handoff of active calls. Soft handoff is characterized by commencing communications with a new BTS site on the same CDMA frequency assignment before terminating communications with an old BTS site. Soft handoffs can be simplified as a “make before break” handoff, where there is no interruption in communications. If soft handoff cannot be achieved, then the result is a “hard” handoff, characterized by a temporary disconnection of the traffic channel. Hard handoffs can be simplified as a “break before make” handoff, where there is a temporary break in communications. Hard handoffs occur when the mobile station is transferred between disjoint active sets, when the CDMA frequency assignment changes, when the frame offset changes, or, less commonly, when the mobile is directed to an analogue voice channel, all of which are less preferred than soft handoffs.